Dictionary of Biblical Words:
Great stress is laid in Scripture on the direction of the eye. A single eye means an eye wholly directed to and occupied with one object: in Scripture this object is Christ. We get the doctrine in Matt. 6, the practice in Phil. 3 in Paul.
Boyd’s Bible Dictionary:
Putting out the eye a warfare custom, especially with dangerous prisoners (Judg. 16:21; 1 Sam. 11:2; 2 Kings 25:7). Painting the eyelids a fashion (2 Kings 9:30; Jer. 4:30; Ezek. 23:40). “Eye-service,” reluctant service (Col. 3:22; Eph. 6:8).